Sheeran vs Gaye - Another Copyright Lawsuit?

Following the lawsuit (failed) over “At the Shape of You,” Ed Sheeran must be getting rather fed up. Now, lawyers are taking him to trial over Thinking Out Loud, claiming it is a rip off of Marvin Gaye’s Let’s Get It On.

He is certainly correct to say that copyright claims are becoming far more frequent.  But why? What has changed over the many years of songwriting, and why is it even a problem in the first place

I intrepidly investigate without actually leaving my comfy chair.